Cholesterol homeostasis is maintained by dietary intake, biosynthesis, metabolism to bile acids, absorption and a process known as reverse cholesterol transport (RCT). Cholesterol is transported in the blood by lipoproteins, which contain different apolipoproteins that are recognized by different receptors on the liver and cells such as macrophages. RCT is involved in the movement of cholesterol from peripheral tissues to the liver for excretion. This pathway may represent up to 70% of the flux of cholesterol to the liver. Inherent in this process is the remodeling of the lipoprotein particles. A key player in RCT is the cholesteryl ester transfer protein (CETP), a glycoprotein that mediates the transfer of cholesteryl ester from the cardioprotective High Density Lipoprotein (HDL) particles to the pro-atherogenic LDL (Low Density Lipoprotein), VLDL (Very Low Density Lipoprotein) and IDL (Intermediate Density Lipoprotein).
CETP is a glycoprotein with a molecular weight of about 74 kDa and a primary sequence containing 476 amino acids. Although the amino acid sequence would suggest the protein to be highly hydrophobic, most of the hydrophobic residues reside mainly on the interior, as the protein is soluble in water (Hesler et al., J. Biol. Chem., 262:2275-2282, 1987). This hydrophobic pocket allows for the binding of neutral lipids (Au-Young and Fielding, Proc. Natl. Acad. Sci., 89:4094-4098, 1992). Using the crystallographic structure of a related protein, BPI (bactericidal/permeability increasing protein) with about 20% homology to CETP, a model of CETP was published by Bruce et al., Curr. Opin. Struct. Biol., 8:426-434, 1998. The C-terminal residues were predicted to form an amphipathic helix that covers the opening of an N-terminal pocket. Lipid transfer is thought to occur through a disordering of the lipids in the lipoprotein surface followed by flipping open of the hydrophobic pocket with entry of the neutral lipid.
CETP facilitates exchange and net transfer of neutral lipids, mainly cholesteryl esters and triglycerides between plasma lipoproteins. Phospholipids can also be transferred to a lesser degree. CETP inhibitors have emerged with the potential to increase HDL cholesterol (HDL-C) to levels exceeding those of the currently available therapies.
In normal human plasma, the CETP concentration is around 1-3 μg/ml; however, in patients with hypercholesterolemia, or mixed hyperlipidemias with hypertriglyceridemia, the CETP concentrations have been reported to be 2-3 fold higher (Marcel et al., Journal of Clinical Investigation, 85:10-17, 1990, and McPherson et al., Arteriosclerosis and Thrombosis: A Journal of Vascular Biology, 11(4):797-804, 1991). Plasma CETP activity is modulated by a variety of factors including: plasma CETP concentration, plasma levels of lipoprotein acceptors and donors, plasma triglyceride levels, physical exercise, alcohol and smoking. Circulating CETP is associated with HDL, VLDL and LDL particles (Nishida et al., Journal of Biological Chemistry, 268(22):16352-60, 1993). Most seems to be associated with HDL and only about 1% is reported to be present in free form.
In patients with Type IIa hypercholesterolemia (familial hypercholesterolemia, LDL-C>160 mg/dL), elevated levels of CETP have been reported as well as increased transfer of cholesteryl ester from HDL to VLDL and LDL (Guerin et al., Arteriosclerosis and Thrombosis: A Journal of Vascular Biology, 14(5):679-85, 1994, and Guerin et al., Arteriosclerosis and Thrombosis: A Journal of Vascular Biology, 14(2):199-206, 1994) thereby generating the smaller more dense LDL particles, which are considered to be atherogenic. Type IV hypertriglyceridemia is characterized by elevated levels of VLDL and VLDL remnants with plasma triglycerides measuring >150 mg/dL. Associated with these elevations are reduced levels of HDL and apoA-I. This may be due to an increase in the CETP-mediated transfer of cholesterol esters to VLDL. This results in the formation of large VLDL1 subfractions, which are the preferential precursors of small dense proatherogenic LDL particles (Packard and Shepard, Arterscler. Thromb. Vasc. Biol., 17:3542-3556, 1997). Type IIB is a mixed hyperlipidemia characterized by simultaneous elevations in both plasma cholesterol and triglycerides with increases in VLDL and LDL and decreases in HDL. The LDL particles are shifted to the small dense LDL 4 and 5 subfractions. Plasma CETP concentrations are elevated and a higher rate of transfer activity has also been reported (Guerin et al., European Journal of Clinical Investigation, 26(6):485-94, 1996). In the case of secondary dyslipidemias such as those found in diabetes, there are also reports of elevated CETP activity particularly in the presence of hypertriglyceridemia (Guerin et al, Arterioscloerosis, Thrombosis and Vascular Biology, 20(1):189-97, 2001).
The first studies with CETP inhibitors were done in rabbits, which express high levels of CETP and are highly susceptible to atherosclerosis when fed a high cholesterol diet. Anti-sense oligonucleotides, antibodies, vaccines and small molecule inhibitors have been tested (Sugano et al., Journal of Biological Chemistry, 273(9):5033-6, 1996; Rittershaus et al., Arteriosclerosis, Thrombosis and Vascular Biology, 20(9):2106-2112, 2000; Whitlock et al., Journal of Clinical Investigation, 84(1):129-37, 1989; and Okamoto et al., Nature, 406:203-207, 2000). These studies showed that inhibition of CETP increased plasma HDL-C levels and particle size as well as decreasing aortic cholesterol content and lesion development. Administration of the small molecule inhibitor JTT-705, which irreversibly inactivated CETP by binding to a crucial cysteine residue (Cys13), to rabbits at a dose of 30 mg/kg inhibited CETP activity, increased HDL-C (+90%), reduced non-HDL-C cholesterol and lesion size (−50% and −70%, respectively, Okamoto et al., Nature, 406:203-207, 2000). However, in another study where rabbits had severe hypercholesterolemia, JTT-705 was not efficacious in preventing lesion development (Huang et al., Clin. Sci., 103(6):587-594, 2002). Interestingly there were significant elevations of plasma triglycerides in this study with JTT-705 treatment. In later clinical studies, JTT-705 was found to raise HDL-C, modestly lower LDL-C and not alter triglyceride levels (DeGrooth et al., Circulation, 105(18):2159-2165, 2002). A more potent CETP inhibitor, Torcetrapib, has shown positive results in Phase II trials, particularly in combination with Atorvastatin (Brousseau et al., New England Journal of Medicine, 350(15):1505-1515, 2004). The references cited herein are hereby incorporated by reference in their entries.
There is a continuing need for new CETP inhibitors. There is a further need for new CETP inhibitors that increase HDL-C, increase the ratio of HDL-C/total cholesterol, increase the ratio of HCL-C/LDL-C, and/or lower LDL-C and/or lower non-HDL-C cholesterol.
It is an object of the present invention to provide compounds that are CETP inhibitors. It is also an object of the invention to provide a method of treating or ameliorating a condition mediated by CETP. It is a further object of the invention to provide a useful pharmaceutical composition comprising a compound of the present invention useful as a CETP inhibitor.